This invention relates to X-ray film cassettes and, more particularly, to an X-ray film cassette capable of providing forces on the film that vary across the surface of the cassette and to a method of making the same.
Much work has been devoted over the years to the making of X-ray film cassettes. There is a particular problem encountered in making such cassettes for medical purposes in that X-ray intensifying screens must be used to accommodate the film to the lower X-ray energy levels used on humans. Intensifying screens fluoresce under X-ray bombardment and thus release light energy to aid in the exposure of the adjacent X-ray film. These screens are placed on the front (and back) sides of the cassette on either side of the film. The film must be clamped tightly in-between these two intensifying screens. If the intensifying screens are not maintained in intimate contact with the X-ray film, the fluorescing light tends to become diffused and the X-ray image blurred.
This problem is not particularly severe until such time as large area films are employed. With such large films, it becomes relatively difficult to maintain the desired pressure profile across the entire surface of the film. Reinforcing materials cannot be used across the faces of the cassette without causing unwanted shielding and shadows within the X-ray image itself. Likewise, thick materials having greater rigidity cannot be used since they are more dense and block the passage of X-rays. A further problem arises with the large area films in that the surfaces tend to trap air, particularly within the center region of the film, when the cassette is closed. This in and of itself causes unwanted separation between the intensifying screen and the film and hence a blurring of the image.
To alleviate many of these problems, certain of the prior art cassettes provided plates with some inward curvature, hoping to alleviate the air pockets and supply sufficient pressure. Exemplary of such cassettes are those described by Canadian Pat. No. 843,519, issued June 2, 1970 to Frederick F. Tone and U.S. Pat. No. 3,153,145, issued Oct. 13, 1954 to Simon Yerkovich.
While cassettes of this type did represent a significant advance over the art, the utilization of curved plates in and of itself did not entirely alleviate the problem. Air could still be trapped; film screen contact variations are noticeable in the film when developed because air entrapment produced undesired regions of blurring.
It is therefore an object of this invention to alleviate many of the disadvantages of the prior art X-ray film cassettes.
Another object of this invention is to provide an improved X-ray film cassette that is particularly useful with large area X-ray films.
A further object of this invention is to provide an improved method of manufacturing X-ray film cassettes.